Analytical Methods

Colilert method for total
coliforms and Escherichia coli.
Updated January 2007

Colilert is a commercially available
enzyme-substrate liquid-broth medium (IDEXX Laboratories, Inc., Westbrook,
Maine) that allows the simultaneous detection of total coliforms and
Escherichia coli (E. coli). It is available in the most-probable
number (MPN) or the presence/absence (PA) format. The MPN method is facilitated
by use of a specially designed disposable incubation tray called the Quanti-Tray®.
The PA method can be done using any sterile bottle of the appropriate size.
Both methods can be done in the field or laboratory.

THEORY:
Two enzyme substrates are included in Colilert—a chromogen that reacts with the
enzyme found in total coliforms (galactosidase), and a fluorogen that reacts
with an enzyme found in E. coli (glucuronidase). After 24 hours
incubation at 35oC, a total- coliform-positive reaction turns the
medium yellow; an E. coli-positive reaction causes the medium to
fluoresce under a long-wave ultraviolet light (366 nm).

USE:
The Colilert method is approved for use with drinking water. It can be used to
monitor all types of water, including water that is high in suspended sediment;
it can also be used to monitor sediments. The PA format is suitable for use in
drinking-water or ground-water studies, when only a presence or absence result
is needed. No special equipment is required to use the PA format. The MPN
format can be used with all waters; however, it requires the use of a special
machine that seals the incubation tray. Quanti-Tray provides counts from 1 to
200 MPN/100 mL. Quanti-Tray/2000 provides counts from 1 to 2,400 MPN/100 mL.
Both tests can provide a wider range by diluting the sample 1:10 or 1:100 with
sterile distilled or deionized water.

The Quanti-Tray sealer (Cat WQTS2X-115)
is purchased from the manufacturer (IDEXX, 800/321-0207).

Standard sample-collection bottles may be
used. Because the Colilert substrate is already buffered, when preparing
dilutions, use sterile distilled or deionized water.

REFERENCES:
American Public Health Association, American Water Works Association, and Water
Pollution Control Federation, 2004, Standard methods for the analysis of water
and wastewater (21st ed.): Washington, D.C. American Public Health
Association, Section 9223.